We propose a series of experiments to describe the biological, genetic and molecular behavior of a class of movable genetic elements in Drosophila melanogaster known as P factors. The discovery of P factors came from their ability to cause, under certain well-defined conditions, a syndrome of germline abnormalities ("hybrid dysgenesis") involving high mutability, chromosome breakage, sterility and other dysgenic traits. Recently, the availability of a P factor as cloned DNA has made it possible to determine the locations of P factors on any given chromosome by the hybridization of labeled P-factor DNA to polytene chromosomes. The experiments we propose will combine the methods of classical Drosophila genetics with in situ hybridization to provide an in depth description of how P factors behave, and how geneticists might use them as tools for cloning other Drosophila genes. As a representative of transposable elements in general, P factors are especially well suited for study. One possibility is that they exist as parasitic or "selfish" DNA. Given the widespread existence of movable genetic elements in higher organisms, possibly including humans, we believe that the study of P factors will prove improtant to understanding environmental mutagenesis. By their transpositional activity, movable elements might be able to insert at the sites of normal genes resulting in unscheduled and pathological activation of the genes. Moreover, the understanding of movable elements is sure to have broad implications for molecular as well as population and evolutionary genetics in general. The work we propose is aimed at answering the following questions: (1) By what mechanisms do P factors cause their various dysgenic traitts, and how is this activity controlled? (2) How do P factors behave on the level of population genetics and evolution? (3) What are the best ways to use P factors as cloning tools? Our long term objective is to understand the behavior of P factors on all levels of organization, and to relate it to a general theory of movable genetic elements.